1. Field of the Invention
The present invention relates to a heat treatment apparatus and method of performing heat treatment on a substrate by irradiating a flashlight onto a semiconductor wafer, a glass substrate or the like (hereinafter briefly referred to as a “substrate”).
2. Description of the Background Art
In an ion activation step for an ion-implanted semiconductor wafer, a heat treatment apparatus such as a lamp annealing apparatus with a halogen lamp employed therein has conventionally been used. Such a heat treatment apparatus heats (anneals) a semiconductor wafer to temperatures ranging, for example, between about 1000 and 1100° C. thereby achieving ion activation in the semiconductor wafer. In such a heat treatment apparatus, the temperature of a substrate is raised at a speed of about several hundred degrees per second by the use of energy of light emitted from the halogen lamp.
However, it has been found out that, in the case of achieving ion activation in a semiconductor wafer using a heat treatment apparatus which raises the temperature of a substrate at a speed of about several hundred degrees per second, a phenomenon takes place in which the profile of ions implanted into the semiconductor wafer becomes gentle, that is, ions are diffused by heat. When such a phenomenon takes place, implanted ions are diffused even if a high concentration of ions are implanted into the surface of the semiconductor wafer, resulting in a problem in that more ions than required need to be implanted.
To solve the above-described problem, the technique for raising the temperature of only the surface of a semiconductor wafer within an extremely short period of time (several milliseconds or less) by irradiating a flashlight to the surface of the semiconductor wafer by the use of a xenon flash lamp or the like is proposed by, for example, Japanese Patent Application Laid-Open Nos. 59-169125 (1984) and 63-166219 (1988). Since there is not enough time for ions to diffuse in the case of raising the temperature of the wafer surface within an extremely short period of time by a xenon flash lamp, ion activation alone can be achieved without the profile of ions implanted into a semiconductor wafer becoming gentle.
A xenon flash lamp instantaneously irradiates light of extremely high energy to a semiconductor wafer, so that the surface temperature of the semiconductor wafer is raised in an instant. When energy of irradiated light exceeds a threshold value, rapid thermal expansion at the surface may cause the semiconductor wafer to break with a high probability. Therefore, in an actual heat treatment, light of energy having a certain degree of process margin lower than the threshold value is irradiated.
However, when heating a semiconductor wafer being held by a susceptor by irradiating a flashlight from a xenon flash lamp, the semiconductor wafer may break even with a flashlight having energy lower than the above threshold value. This is because, when the semiconductor wafer is going to warp to be convex due to rapid thermal expansion at the surface resulting from instantaneous flashlight irradiation, a great force is applied to a contact part, if any, between an end of the wafer and a pocket edge of the susceptor or positioning pin, whereas there is not sufficient time for the wafer to move sliding over the susceptor in order to relieve such stress. As a result, even when irradiating a flashlight having energy lower than the above-mentioned threshold value, stress generated at a contact part, if any, between an end of the semiconductor wafer and something at the time of instantaneous thermal expansion causes the semiconductor wafer to break.